The present invention relates to a projection type image display apparatus which forms an optical image based on a picture signal by means of an image display element and projects the optical image on a screen or the like.
With a conventional projector optical system, when optical modulation of the image display element is performed in order to minimize the luminance, a phenomenon called “grayish black” occurs with which the light absorbed by a light-exiting polarization plate is not sufficient and the screen luminance does not decrease. The light-exiting polarization plate aligns the polarization of light beams modulated by the image display element.
For this reason, there is a means for improving the contrast by reducing the minimum luminance of a projection type image display apparatus through light control means for changing the light volume of the entire screen based on external signals, other than a light valve. External signals in this case include a picture signal, an external environment measurement signal, a signal intentionally manipulated by user, etc. As one example of the above means, techniques using a light-masking unit for changing the amount of masking light according to an image signal in an illumination optical system are disclosed in U.S. Patent Application Publication No. US 2003/0086265A1, Japanese Patent Laid-open No. 2005-17500 and Japanese Patent Laid-open No. 2005-31103.